We previously found that insulin rapidly suppresses the activity of a soluble PTPase in skeletal muscle of insulin-sensitive subjects, but not in subjects resistant to insulin and that activity of a particulate PTPase is chronically elevated in muscle of resistant subjects. Either of these abnormalities could be responsible for insulin resistance. We have confirmed the rapid suppression of PTPase activity by insulin in cultured cells and in rabbit skeletal muscle in vivo. The insulin suppressed PTPase can be resolved by ion-exchange chromatography. As part of our effort to understand the contribution of PTPases to insulin resistance, we identified PTPase cDNAs in human skeletal muscle using PCR. The 5 major PTPases amplified included PTP1B, TCPTP, the band 4.1 - related PTPases, PTPH1 and PTP-MEG, and a novel putative PTPase. We have isolated a full- length cDNA and expressed the catalytic domain of this novel PTPase, which we call PTP-PEST because its C-terminal segment is rich in Pro, Glu/Asp and Ser/Thr and contains potential PEST sequences, which are found in proteins with very short intracellular half lives. PTP1B and PTP-PEST RNA species increase 3-5 fold in response to insulin, suggesting that these PTPases may contribute to long-term effects of the hormone.